Production of phosphate coatings on metal surfaces



Patented May 22, 1951 UNITED PRODUCTION OF PHOSPHATE COATINGS ON METAL SURFACES Royston Fraser Drysdale, Croydon, England, as-

signor to The Walterisation Company Limited,

Croydon, England No Drawing.

Application June 22, 1948, Serial No. 34,568. In Great Britain April 25, 1947 7 Claims. 1

The production of protective corrosion-resistant coatings on metal surfaces by treatment of the metal with a dilute solution of phosphoric acid or a metal phosphate is well known. Iron or steel and zinc are the metals most commonly treated, and common examples of treating solutions are dilute solution of phosphoric acid with iron phosphate, manganese phosphate or zinc phosphate. Coatings formed in this way are efficient bonds for organic finishes such as paints, enamels or lacquers and are of considerable commercial importance.

The formation of such coatings involves treating the metal surface with the solution for a period of time which may extend to several hours. Various attempts have been made to reduce this time of treating and thereby increase the emciency of the process. One such proposal has been the addition to the phosphating bath of phosphates of metals less basic than the metal being treated. For example, copper phosphate may be added to a bath in which iron or steel is being treated and gives a reduction in treating time.

It has now been discovered that remarkable reductions in treating time associated with phosphate coatings of excellent quality may be obtained by the incorporation in the phosphating solution of very small quantities of at least two metals one of which is more basic and the other less basic than the metal being treated. The basicity of metals as used throughout the specification and claims is that given in Mellors table of the electro-chemical series of metals as given in Mellors Comprehensive Treatise on Inorganic and Theoretical Chemistry, volume I, page 1014 (1922 edition).

The present invention accordingly comprises a process for the production of corrosion-resistant coatings on metals or alloys by treatment with an aqueous solution containing the acid radical of phosphoric acid, wherein there are present in the solution small quantities of the cations of at least two metals, one more basic and the other less basic than the metal being treated, the total amount of such cations not exceeding 0.04 gm. per litre of the said aqueous solution.

Since it is generally undesirable to have the anions of mineral acids present in the bath to any significant extent, it is theoretically sounder to add the metals to the bath in the form of their phosphates. Since, however, only extremely small quantities are involved, satisfactory results are obtained in practice using other soluble salts, e. g. chlorides or nitrates.

While the invention in its simplest form envisages the addition of two metals to the bath, it is found that even better results are obtained by adding three, four or more cationic metals, e. g. one more basic and two less basic; two more basic and one less basic, or two more and two less basic.

It has been found by experiment that the ratios and total amounts of the added metal ions needed to give optimum results are critical for each set of metals as will be shown in more detail later. Although no theoretical explanation has yet been Worked out, it seems possible that the added metals may in some way influence the electrode potential of the metal surface under treatment.

To illustrate the process of the invention, the effects of adding various amounts of cationic zinc, cobalt and nickel to a ferrous phosphate concentrate, subsequently diluted twenty-five times to form a treating bath used for the treatment of a steel sheet, are described below.

When keeping the concentration of cobalt and nickel constant at 0.017 and 0.114 gm./litre respectively, it was found that with no zinc present the treating time (shown by the cessation of gassing of the sample) was about 45 minutes, whilst with a zinc concentration of 0.048 gm./litre it dropped to 15 minutes. When the zinc concentration was raised to 0.168 gm./litre, the time increased to 23 minutes, and with a concentration of 0.480 gm. per litre it dropped to 12 minutes. Increasing the zinc concentration above this limit caused a steady increase in the treating time. When treating time is plotted against zinc concentration, the curve shows two distinct minima at the zinc concentrations of 0.048 gm./ litre and 0.480 gin/litre mentioned above.

If new the zinc and cobalt concentrations are kept constant at 0.048 gm./litre and 0.017 gm./litre respectively and the nickel concentration is varied, it is found that the treating time with no nickel is erratic and of the order of 1 /2 hours which is an improvement over the treating time for the plain iron phosphate bath which is 2 hours or more. With 0.074 gm./litre of nickel the time dropped to 18 minutes, 0.19 gm./litre of nickel raised it to 35 minutes, and 0.24 gm./litre of nickel lowered it to 18 minutes again.

Similar tests with cobalt, maintaining zinc and nickel constant at 0.048 and 0.074 gm./litre respectively, gave a time of 35 minutes for no cobalt, 15 minutes for 0.02 gm. cobalt and 34 minutes for 0.07 gm. cobalt per litre.

establishes that the phenomenon is a function of the actual concentration of the three metals and not only of their relative proportions.

While no fixed limits can be given, it will be clear that the amounts of metal ions addedto the phosphating bath are extremely small, and far less than the amounts of various additives which have been customarily addedin the prior art. Thus, when using zinc, cobalt and nicke l the total amount of added metal is between 0.10 and 0.15 gm. per litre. of concentrate, subsequently diluted twenty-five times to form'the phosphating bath, and it seems probable that the total amounts of any metals to be added to the concentrate will be less than 1 gm./1itre and probably less than 0.5. gm./litre. In some cases, of course, the maximum concentration of any given metal will be limited by the solubility of its phosphate in phosphoric acid.

A seriesof experiments were carried out using the molar equivalent of magnesium, aluminium, barium, strontium,manganese, calcium and chromium, as the more basicmetals in place of the zinc, in the zinc-.cobalt-nickel mixture mentioned above. Similar improvements in treating time were obtained, but it was found that these were not the maximum obtainable when using these alternativemetals with cobalt and nickel. Thus, for example, the addition of half the molar equivalent of magnesium was found to be more effective than the addition of the molar equivalent, which in turn was more effective than twice the molar, equivalent. In other words, the addition to. a. litre of concentrate of 0.017 gm. ofcobalt, :074gm. of nickel and 0.009 gm. of magnesium was more eiTective than when the magnesium concentration was increased to 0.018 gm. and further increase to 0.036 gram still further reducedthe effect.

It is possible that the optimum relative proportions and absolute concentrations of any particular combination of metallic ions in the solution may be a function of their mobilities or of the ease with which they can be adsorbed by a phosphate layer. Sufiicient data is not yet available to substantiate this, however.

Whatever thetrue explanation of the effect of these very small additions, it can be shown that their effect is due to a different mechanism from that of the much larger amounts of various additions which have been made in the prior art. This is indicated by the loss of the acceleration eifect obtained by the addition of larger quantities as already shown in the cases of cobalt, nickel, zinc and magnesium, and is even more strikingly indicated in the case of other metals where the addition of the small amounts quoted has a definite effect in shortening the treating time, whereas addition of amounts of the same metallic ions comparable with those customarily added in the prior art produce a deceleration or even completely inhibit the production of an adherent phosphate layer upon ferrous articles treated in the bath. For example, the addition of 0.00076 gm. of aluminium per litre of treating solution to asolution contaim'ng 0.00068 gm. of cobalt and 0.00296 gm. of nickel per litre of treating solution resulted in a reduction in the treating time of 29% of that previously required, whereas the addition of 0:076 gm. of aluminium per litre to the same solution resulted in the complete inhibition of the formation of an adherent phosphate film.

Another series of experiments showed that the analogous substitution of the molar equivalent ofammonium, sodium, or potassium for the zinc was virtually without effect on the treatment time. A slight decrease in treatment time wasobserved butthis was not comparable with that produced by metals forming insoluble phosphatesand was. of such an order as to be conceivably within the limits of experimental error. The alkali metals are thus excluded from the scope of the present invention.

In order to substantiate the assertion that it is essential to have at least one metal more basic and, one less. basic than iron present, a series of experiments were carried out to determine the effect of, additionsof minor quantites of the cations of some, of the more basic metals already mentioned Thus, amounts of the same order as those already referredto, of zinc, calcium, manganese andmagnesium were added to a ferrous phosphate bath in varying. combinations and ratios. In no case was any significant alteration in the treatment time observed.

The advantages of the invention willbe read: ily apparent from the following examples, in. all of which therewa employeda bath prepared from a concentrate made bydissolving grams of ferrous phosphate and 15 grams of iron.filings in 20 0. cc s, concentrated phosphoric acid and 800 cos. of water. This; concentrate was diluted togive a bath of 30, points strength, (the pointsstrength is the number of ccs, of decinormal causticsoda needed to neutralise 10 cos. of the bath solution, usingphenolphthalein as, an indicator), which was .operated at 98 C. Test panels were cut from a single sheet oi high lustre cold rolled steel sheet. These test panels were first degreased and thenimmersed in'the bath without further pretreatment.

Example 1 .S,tandard for comparison The bath; as described above was employed without the addition, of any metal additives.

The treatinggtime, asmeasured to the cessation.

ofevolution of hydrogen .wasll l minutes.

Example 2 In this case the additions to the concentrate were 0.047 gm./litre ofgzinc, 0.074 gm'./litre of nickel and 0.025 gm./litre of cobalt. The treating time was reduced to 30 minutes, i. e. only 26.2 of that required in Example 1.

Example 4 To'aconcentrate as used. in Example 3, there was'also added 0.0l8-gm./litre of magnesium. The treating time was 24 minutes, 'i. e. only 21% of that required iii-Example l.

A list of solutions which have been found experimentally to be of-great value, giving an average treating time ofthe order of one third-of that needed when no additives are---present-,= isare indicated as and those more electro-' negative as ions are also Solution A:

0.048 gm. 0.074 gm.

Solution B:

0.04:8 gm. 0.017 gm. 0.074 gm.

Solution C:

0.009 gm. 0.017 gm. 0.074 gm.

Solution D:

0.019 gm. 0.025 gm. 0.074 cm.

Solution E:

). Total weights of addedmetal indicated.

Grams zinc .1 nickel 91 zinc cobalt }0.139 nickel magnesium cobalt 0.100 nickel aluminium cobalt 0.118 nickel A zinc lead 0.262

0.0393 gm. copper Solution F:

0.0169 gm. zinc 0.0183 gm. nickel 0.0030 gm. cobalt 0.0393 gm. copper Solution G:

0.098 gm. 0.074 gm. 0.025 gm.

Solution H:

0.063 gm. 0.017 gm. 0.074 gm.

Solution I:

0.029 gm. 0.017 gm; 0.074 cm.

Solution J 0.018 gm. 0.175 gm. 0.0393 gm. copper Solution K:

0.041 gm. 0.025 gm. 0.074 gm.

Solution L:

0.039 gm. 0.017 gm. 0.074 gm.

Solution M:

0.047 gm. 0.029 gm. 0.074 gm. 0.025 gm.

Solution N:

0.048 gm. 0.018 gm. 0.074 gm. 0.025 gm.

From the barium nickel 0.197 cobalt strontium cobalt 0.154 nickel calcium cobalt 0.120 nickel magnesium lead 0.232

cobalt 0.140

manganese nickel chromium cobalt 0.130 nickel zinc calcium nickel 0175 cobalt zinc magnesium nickel 0165 cobalt foregoing it will be seen that the 6 is less than 0.5 gm./litre, the corresponding bath concentrations being 0.04 and 0.02 gm./litre.

Apart from the fact that alkali metals are inefiective and therefore outside the scope of the invention, there are a few other elementary factors which should be considered. Thus materials which would precipitate some or all of the added. metals should be absent from the bath. Thus,. copper or other metals in group II of the qualitative analysis table should not be used with a phosphating bath containing hydrogen sulphide.

The invention also includes phosphating baths for use in the process, such baths containing the acid radical of phosphoric acid and the metal cations as previously described. The bath strength is desirable 30 points, but a margin of 4 or 5 points either way is permissible. Other bath strengths may however be used, as is known in the art, in which case the additions to the concentrate are adjusted to maintain the total amounts of added metal in the treating solution within the limits specified herein. In the concentrates the relative proportions of the ingredients will obviously be the same as in the bath, the conventional degree of concentration being 25 times as already stated. Other degrees of concentration are obviously possible.

Furthermore, although the invention has been described with reference to a ferrous phosphate bath, good results were obtained with zinc phosphate baths and manganese phosphate baths which are commonly used alternatives. Thus the addition of alkaline earth metals, especially calcium, together with cobalt and nickel (of. solution I) to a zinc phosphate bath was found most effective.

I claim:

1. A process for the production of corrosionresistant coatings on a metal selected from the group which consists of zinc and iron, which comprises treating said metal in an aqueous solution which contains essentially at least one: primary orthophosphate of a metal selected from: the group which consists of zinc, manganese and. iron, there being also present in said aqueous solution at least a very small amount of the cations of at least two metals, neither of said metals being an alkali metal and one of said metals being more basic than the metal being treated, and the other of said metals being less basic than the metal being treated, the total amount of said cations present in said solution ranging from 0.003 to about 0.04 gram :per litre of the said aqueous solution, said cations being present as part of a Water-soluble salt of said metals, or as substances which form water-soluble acid phosphates.

2. A process for the production of corrosionresistant coatings on metal as defined in claim 1, wherein the total amount of said cations present in said aqueous solution does not exceed 0.02 gram per litre of said aqueous solution.

3. A process as defined in claim 1, wherein, the total amount of said cations present in said; aqueous solution ranges from 0.003 gram to 0.0l

gram per litre of said aqueous solution.

and the less basic metal includes at least oneear-3&6

metal selected from the 'groupwhich' consists of nickeLcobalt; leadjand copper. v

5. An impfovedphosphating bath for thetreat ment of metallic surfaces; comprising an aqueous solutioncon'taining essentially at least. one pri-' mary metal orthophosphate of a metal selected froni'the group which consists of zinc, manganese and iron, said solution containing at least a very small amount of the cations of at least two metals other than alkali meta1s, one off said twometals being more basic and the "other less basic-than a metal selected'from the group consisting of iron and zinc, the" total amounts of said cations being comprised Within a range ex tending from 0.003 gram perlitre to 0.04 gram per litre of said aqueous solution, said cations beingpresent asionic'eonstituents of salts which aresoIubIefinsaid aqueous solution.

6. Animproved phosphating bath as defined in 019311 "f'wiiiiii tfi t'otai aniuht of sai'd cati ons presentldoes not exceed 0.02 gram'per litre of li' ol tmn- 7 A improyed 'phosphating bath as defined claim'fi wherein the total amount of said cations.

ofsaid two metals does notexceed 0.01 gram per lit ie Of bath solution, ROYSTON FRASER DRYSDALE.

REFERENCES CITED The-following references are of record in the file of this patent:

UNITED s'r ES PATEN'rs Date 

1. A PROCESS FOR THE PRODUCTION OF CORROSIONRESISTANT COATINGS ON A METAL SELECTED FROM THE GROUP WHICH CONSISTS OF ZINC AND IRON, WHICH COMPRISES TREATING SAID METAL IN AN AQUEOUS SOLUTION WHICH CONTAINS ESSENTIALLY AT LEAST ONE PRIMARY ORTHOPHOSPHATE OF A METAL SELECTED FROM THE GROUP WHICH CONSISTS OF ZINC, MANGANESE AND IRON THERE BEING ALSO PRESENT IN SAID AQUEOUS SOLUTION AT LEAST A VERY SMALL AMOUNT OF THE CATIONS OF AT LEAST TWO METALS, NEITHER OF SAID METALS BEING AN ALKALI METAL AND ONE OF SAID METALS BEING MORE BASIC THAN THE METAL BEING TREATED, AND THE OTHER OF SAID METALS BEING LESS BASIC THAN THE METAL BEING TREATED, THE TOTAL AMOUNT OF SAID CATIONS PRESENT IN SAID SOLUTION RANGING FROM 0.003 TO ABOUT 0.04 GRAM PER LITRE OF THE SAID AQUEOUS SOLUTION, SAID CATIONS BEING PRESENT AS PART OF A WATER-SOLUBLE SALT OF SAID METALS, OR AS SUBSTANCES WHICH FORM WATER-SOLUBLE ACID PHOSPHATES. 